dns on master
@ IN MX mailserver.domain.com
mailserver1 IN NS mailserver1.domain.com.
mailserver2 IN NS mailserver2.domain.com.
mailserver1 IN A 10.x.x.1
mailserver2 IN A 10.x.x.2
then on each of those mailservers setup an domain record for mailserver.domain.com and define @ IN A to the machines ip
this is because NS is the only DNS method that checks to see if a machine is up with out hard failing and will query diffrent dns servers at the same time providing fail over and load balancing